This invention relates to a method for preparing catalysts useful in the manufacture of dicarboxylic acid anhydrides by the oxidation of hydrocarbons. More particularly, it is directed to the preparation of high yield catalysts suitable for producing maleic anhydride from saturated hydrocarbons at lower temperature than heretofore thought possible.
Maleic anhydride is used alone or in combination with other acids in the manufacture of alkyd and polyester resins. It is also a versatile intermediate for chemical synthesis. Significant quantities of maleic anhydride are produced each year to satisfy these needs.
U.S. Pat. No. 3,293,268 teaches a process of oxidizing saturated aliphatic hydrocarbons to maleic anhydride under controlled temperature conditions and in the presence of phosphorus-vanadium-oxygen catalysts. A method of preparing such catalysts involved reacting phosphoric acid with a vanadium compound in aqueous hydrochloric acid solution, recovering the remaining solids by evaporating the solution to dryness, and then heating the solids to 300.degree. to 500.degree. C. The resulting catalysts were ground to pass a 20 mesh screen and pelleted to form tablets.
U.S. Pat. No. 3,915,892 teaches a three phase transition in which a mixed oxide of vanadium and pentavalent phosphorus in substantially the dihydrate form is subjected to:
(1) a first heat step at 370-394.degree. C. for about 0.5-2 hours wherein the dihydrate is converted to a monohydrate; PA1 (2) a second heat treatment at 395-425.degree. C. wherein the monohydrate is converted to the anhydrous oxide; PA1 (3) a third heat step for post dehydration bulk crystal phase transition at 450-520.degree. C. at from about 4-16 hours.
In the third phase, a mixture of air and butane is used.
One of the disadvantages of this process is that it contemplates temperatures above which are most desirable in the conversion of a hydrocarbon such as butane to maleic anhydride, and therefore makes it difficult if not impossible to perform the last treatment phase in most maleic anhydride converters which are not made to withstand such temperatures. A solution to this problem, and any method for the preparation of the catalyst which increases its yield would be a significant advance in the art and are objects of this invention.